Over the past fifteen to twenty years, micro mirror array based spatial light modulator (SLM) has made many incremental technical progresses and gained great acceptance in the display industry. The devices operate by tilting individual micro mirror plate in the array around a torsion hinge with an electrostatic torque to deflect the incident light to a predetermined exit direction. In a more popular digital mode operation, the directional light is turn “on” or “off” by rotating selectively the individual mirrors in a micro mirror array and mechanically stopped at a specific landing position to ensure the precision of deflection angles. A functional micro mirror array requires low contact sticking forces at the mechanical stops and high efficiency of electrostatic torques to control timing, to overcome surfaces contact adhesions, and to ensure the robotics and reliability. A high performance spatial light modulator for display application produces high brightness and high contrast ratio videos images.
Early SLM in video application suffers a disadvantage of low brightness and low contrast ratio of the images projected. Previous SLM design typically has a low active reflection area fill-ratio of pixels (e.g., ratio between active reflective areas and non-active areas in each pixel). A large inactive area around each pixel in the array of SLM results to a low optical coupling efficiency and low brightness. The scattered light from these inactive areas in the array forms diffraction patterns adversely impact the contrast of video images. Another major sources reducing the contrast ratio of micro mirror array based SLM is the diffraction of the scattered light from two straight edges of each mirror in the array that are perpendicular to the incident illumination. In a traditional square shape mirror design, an orthogonal incident light is scattered directly by the perpendicular straight leading and trailing edges of each mirrors in the array during the operation. The scattered light produces a diffraction pattern and much of the diffracted light is collected by the projection lenses. The bright diffraction pattern smears out the high contrast of projected video images.
One type of micro mirror based SLM is the Digital Mirror Device (DMD), developed by Texas Instruments and described by Hornbeck. The most recent implementations include a micro mirror plate suspended via a rigid vertical support post on top of a yoke plate. The yoke plate is further comprised a pair of torsion hinges and two pair of horizontal landing tips above addressing electrodes. The electrostatic forces on the yoke plate and mirror plate controlled by the voltage potentials on the addressing electrodes cause the bi-directional rotation of both plates. The double plate structure is used to provide an approximately flat mirror surface that covers the underlying circuitry and hinge mechanism, which is one way in order to achieve an acceptable contrast ratio.
However, the vertical mirror support post which elevated the mirror plate above the hinge yoke plate has two negative impacts on the contrast ratio of the DMD. First, a large dimple (caused by the fabrication of mirror support post) is present at the center of the mirror in current designs which causes scattering of the incident light and reduces optical efficiency. Second, the rotation of double plate causes a horizontal displacement of mirror reflective surfaces along the surface of DMD, resulting a horizontal vibration of a micro mirror during operation. The horizontal movement of mirrors requires extra larger gaps to be design in between the mirrors in the array, reducing the active reflection area fill-ratio further. For example, if the rotation of mirror on each direction is 12°, every one micron apart between the mirror and the yoke resulting a 0.2 microns horizontal displacement on each direction. In other words, more than 0.4 microns extra gap spacing between the adjacent mirrors is required for every one micron length of mirror support post to accommodate the horizontal displacement.
The yoke structure has limited the electrostatic efficiency of the capacitive coupling between the bottom electrodes and the yoke and mirror. Especially in a landing position, it requires a high voltage potential bias between the electrodes and the yoke and mirror to enable the angular cross over transition. Double plate structure scatters incident light which also reduces the contrast ratio of the video images.
Another reflective SLM includes an upper optically transmissive substrate held above a lower substrate containing addressing circuitry. One or more electrostatically deflectable elements are suspended by two hinge posts from the upper substrate. In operation, individual mirrors are selectively deflected and serve to spatially modulate light that is incident to, and then reflected back through, the upper transmissive substrate. Motion stops may be attached to the reflective deflectable elements so that the mirror does not snap to the bottom control substrate. Instead, the motion stop rests against the upper transmissive substrate thus limiting the deflection angle of the reflective deflectable elements.
In such top hanging mirror design, the mirror hanging posts and mechanical stops are all exposed to the light of illumination, which reduces the active reflection area fill-ratio and optical efficiency, and increase the light scattering. It is also difficult to control the smoothness of reflective mirror surfaces, which is sandwiched between the deposited aluminum film and LPCVD silicon nitride layers. Deposition film quality determines the roughness of reflective aluminum surfaces. No post-polishing can be done to correct the mirror roughness.
In this invention, a high contrast spatial light modulator for display and printing is fabricated by coupling a high active reflection area fill-ratio and non-diffractive micro mirror array with a high electrostatic efficiency and low surface adhesion control substrate.